Along with the recent spread of portable electronic devices such as portable video cameras or portable PCs, a demand for secondary batteries as movable power sources has rapidly increased. Moreover, such secondary batteries are severely demanded to be smaller and lighter, and have a higher energy density.
Typically, mainstream secondary batteries that can repeat charge and discharge are lead batteries, nickel-cadmium batteries, and the like. These batteries are excellent in charge/discharge characteristics but not enough as movable power sources for portable electronic devices in terms of the battery weight or energy density.
To overcome the situation, research and development are actively made on lithium secondary batteries, as secondary batteries, in which lithium or a lithium alloy is used for a negative electrode. Lithium secondary batteries have excellent characteristics including high energy density, low self-discharge, and light weight.
An electrode of a lithium secondary battery is commonly formed by kneading an active material and a binder together with a solvent for dispersion of the active material to prepare slurry, applying the slurry to a current collector by, for example, a doctor blade method, and drying the applied slurry to form a thin film.
At the present, fluororesins typified by polyvinylidene fluoride (PVDF) are most widely used as binders for electrodes (negative electrodes) of lithium secondary batteries.
Fluororesins however swell with electrolytes when used as binders to cause peeling at an electrode interface in a long-term cycle, lowering the battery characteristics.
As an aqueous binder, carboxymethyl cellulose or the like is used. In a case where carboxymethyl cellulose is used, the flexibility of the resin is insufficient. In such a case, the effect of binding the active material may be insufficient or the adhesion force to the current collector may be markedly lowered.
Patent Literature 1 teaches that the use of a low-crystalline carbon having a graphite interlayer distance (d002) of 0.345 to 0.370 nm as a negative-electrode active material, a styrene-butadiene copolymer (SBR) as a binder, and carboxymethyl cellulose as a thickener can provide a favorable negative electrode, leading to production of a battery excellent in output characteristics.
However, also in the case where SBR is used as a binder, the binder swells with an electrolyte to cause peeling at an electrode interface, lowering the battery characteristics.